Interfacial solar steam generators enable sustainable freshwater production via photothermal conversion, yet effective purification of both feed water and distillate remains challenging. Here, a mechanically interlocked MXene/boron-doped mesoporous g-C₃N₄ nanosphere (BMNS) hybrid membrane is engineered through vacuum-assisted self-assembly, synergistically coupling ultraefficient photothermal evaporation with robust photocatalytic degradation. The BMNS nanospheres embedded in loosely stacked MXene nanosheets establish rapid water transport nanochannels while achieving ultralow thermal conductivity (0.43 W·m⁻¹·K⁻¹). This structure enabled a high solar evaporation rate of 2.10 kg·m⁻²·h⁻¹ and achieved a photothermal conversion efficiency of 98.1%. Simultaneously, the generated reactive oxygen species (·OH/·O₂⁻) degraded 95.6% of the organic pollutants (10 ppm Rhodamine B) within 4 h. Furthermore, it maintained a stable evaporation rate of 2.00 kg·m⁻²·h⁻¹ over a long-term operation of 200 h. Field validation using eutrophic lake water demonstrates concurrent clean water production and comprehensive purification: 90.0% total organic carbon removal, 92.0% chemical oxygen demand reduction, 93.1% microbial inactivation, improved optical clarity, and > 99% antibacterial efficiency against Escherichia coli and Staphylococcus aureus. This work provides a scalable blueprint for multifunctional membranes addressing water scarcity and pollution simultaneously.